推荐回答（1个）
Ⅰ 基本介绍
IEEE 802.11 是一种针对无线区域网路(WLAN)所制定的国际性标准，它定义了多
媒体处理控制(Medium access control，MAC)子阶层，包括MAC 管理协定和服
务，以及1和2Mbps等三种低速实体层(PHY)的操作。近年来，随著无线用户兴
日俱增以及对高速多媒体服务兴起，对频宽需求也随之增加。
为支援新一代无线系统。目前802.11标准已开发出一种在5GHz U-NII频段中执
行的高速实体层IEEE 802.11a PHY标准。该标准采用OFDM调变方式，可提供
八种不同资料传输率，其范围从6Mbps到54Mbps。
除了比较进阶调变系统与现有的低速PHY 以外，802.11a PHY 的另一项重点是
1/2速率的回旋码(convolutional code)，其中还包含了实体层针对改善讯框的传输
能力。然而，802.11a 的PHY 必须工作在最初为低速PHY 所设计的MAC 协定
中，因此，对目前的MAC协定来说，增加link adaptation能力是相当重要的。
在此同时，「产量(Goodput)」也关系到用户实际上所接收的频寛。为分析802.11
无线区域网路，Goodput 采用了Wireless LAN 无线晶片组。然而，由於高速OFDM
PHY在此时间内是无效的，所以我们假设QPSK调变没有前向纠错(Forward error
correction，FEC)能力，同时分析了MSDU (MAC service data units)之不同大小的
Goodput效能。而此单位是由IEEE 802.2逻辑连结控制(Logical Link Control，LLC)
子阶层所产生的。实际上，在802.11 标准中即指出，MSDU 能够更进一步分裂
成较小的MAC 讯框。换句话说，也就是专门应用在传输的MAC 协定资料单位
(MAC protocol data units，MPDU)。
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